Cardiac pacing system with improved end-of-life detector

Abstract

A pacemaker is provided having an improved circuit for obtaining a measure of battery end-of-life. The EOL detector includes an input stage with a current mirror output providing a current which is proportional to the current being consumed by the pacemaker circuitry, and thus being expended by the battery. The proportional current is directed to charge a capacitor, which is periodically discharged at a constant current, following which the battery goes through repeated cycles of charging and discharging. A measuring circuit measures the time of capacitor discharge, thereby providing a measure of battery energy discharge during the interval that the capacitor was charging. The interval counts are accumulated to provide an overall count representative of battery expenditure, and thus of pacemaker EOL.

What is claimed is: 
     
       1. A pacemaker having a battery, pacemaker circuitry for carrying out pacing functions, and an energy consumption detector, said detector comprising (a) accumulating means for accumulating a signal representative of the integral of pacemaker circuitry current over the time since it last started to accumulate said signal;   (b) readout means for reading out said accumulated signal when it reaches a predetermined threshold and for developing therefrom a measure of battery expenditure over said time;   (c) said readout means having means for zeroing out said accumulated signal so that said accumulating means repeats the function of accumulating a said representative signal from the time of said zeroing out until the time of the next readout; and   (d) means for accumulating signals which have been read out and for storing a signal representative of overall battery expenditure.   
     
     
       2. The pacemaker as described in claim 1, wherein said accumulating means comprises sense means for continuously sensing a measure of current flow through said pacemaker circuitry. 
     
     
       3. The pacemaker as described in claim 1, wherein said accumulating means comprises a capacitor and charging means for charging said capacitor with a current proportional to said pacemaker circuitry current. 
     
     
       4. The pacemaker as described in claim 3, wherein said readout means comprises means for discharging said capacitor to a predetermined value and for developing a series of pulses during the time of discharge of said capacitor, said series of pulses representing a measure of the energy discharged from said capacitor. 
     
     
       5. The pacemaker as described in claim 4, wherein said readout means comprises means for initiating readout of said capacitor when the voltage on said capacitor reaches a first threshold, and means for terminating readout of said capacitor when the voltage on said capacitor reaches a second threshold. 
     
     
       6. The pacemaker as described in claim 3, wherein said readout means comprises discharge means for discharging said capacitor at a constant discharge current. 
     
     
       7. The pacemaker as described in claim 1, wherein said accumulating means comprises a current sense means for sensing the pacemaker circuitry current, in combination with a circuit for providing a current having a predetermined linear relation to said sensed current. 
     
     
       8. The pacemaker as described in claim 1, in combination with means for inputting pacemaker operating parameter data, means for obtaining from said operating data an estimate of the rate at which said pacemaker is currently using energy from said battery, and means for determining a measure of end of life of said pacemaker from said estimate and from said stored signal representative of overall battery expenditure. 
     
     
       9. An implantable pacemaker system having a battery for the supply of energy, a pacing subsystem connected to said battery for generating and delivering pacing pulses to a patient, and a circuit for obtaining a measure of battery discharge, said circuit comprising, sense means for generating a current proportional to the current through said pacemaker subsystem;   a capacitor connected to receive said proportional current;   discharge means for cyclically discharging said capacitor;   measure means for determining a measure of the energy discharged from said capacitor each time it is discharged and for generating signals representative of said measure, said signals thus being representative of battery discharge during each charging of said capacitor; and   accumulating means for accumulating said representative signals, thereby providing a measure of overall battery discharge.   
     
     
       10. The pacemaker as described in claim 9, wherein said discharge means comprises a trigger circuit which triggers when the voltage on said capacitor rises to a predetermined level. 
     
     
       11. The pacemaker as described in claim 9, wherein said discharge means comprises means for discharging said capacitor at a constant discharge current. 
     
     
       12. The pacemaker as described in claim 11, wherein said measure means comprises a pulse generator having a period between pulses much greater than the time of discharge of said capacitor, and count means for counting said pulses during said time of discharge. 
     
     
       13. An implantable pacemaker system having a battery for the supply of energy, a pacing subsystem powered by said battery for generating and delivering pacing pulses to a patient, and a circuit for obtaining a measure of the energy discharge of said battery, said circuit comprising: a) a sense circuit for sensing current through said pacemaker circuitry, a capacitor and a charging circuit connecting said sense circuit and said capacitor for charging said capacitor with a charge which is a measure of the integral of said sensed current over time, and thus of battery energy discharge while said capacitor is charging;   b) discharge means for discharging said capacitor at a fixed discharge current, whereby the time of discharge is a measure of said integral and independent of the value of said capacitor; and   c) a measuring circuit for developing a representative signal representative of said discharge time and thus representative of said integral.   
     
     
       14. The pacemaker system as described in claim 13, further comprising cycle means to continuously enable said charging and said discharging, and means to accumulate said representative signals, thereby providing a measure of overall battery discharge. 
     
     
       15. A device having a battery for the supply of energy, a subsystem connected to said battery for performing predetermined functions, and a circuit for obtaining a measure of battery discharge, said circuit comprising, sense means for generating a current proportional to the current through said subsystem;   a capacitor connected to receive said proportional current;   discharge means for cyclically discharging said capacitor at a constant discharge current;   signal means for generating signals representative of the energy discharged from said capacitor each time it is discharged, said signals thus being representative of battery discharge during each charging of said capacitor; and   accumulating means for accumulating said representative signals, thereby providing a measure of overall battery discharge.   
     
     
       16. The device as described in claim 15, wherein said signal means comprises a pulse generator having a pulse rate with a period which is short relative to the time of discharge of said capacitor. 
     
     
       17. The device as described in claim 16, wherein said pulse generator has a rate in the range of about 500 Hz to 5000 Hz.